METHODS OF GAS WARFARE 6 



trench line is very irregular it is likely that gas will 

 into a portion of one's own trenches. The limits oi 

 safety in wind direction are thus determined by the direc- 

 tion of the lines of the trenches. The Germans use a 40 

 angle of safety; that means that on a given straight por- 

 tion of the front the wind direction must lie between the 

 two directions which make angles of 40 with the neigh-: 

 boring sections of the front. The most suitable type of 

 country is where the ground slopes gently away from. 

 where the gas is being discharged. The Germans made 

 one mistake in believing that hilly or wooded country 

 would not do. This was refuted by the French, who made 

 a successful gas attack in hilly and wooded country in 

 the Vosges, as admitted in a captured German report. If 

 the country is flat like that about Ypres, and the wind di- 

 rection is right, there is very little difficulty about making 

 an attack, especially if the enemy does not know anything 

 about it. The element of surprise is important. 



German gas attacks are made by two Regiments of 

 Pioneers, with highly technical officers, including engi- 

 neers, meteorologists, and chemists. They brought their 

 first cylinders into the line without our knowing anything 

 about it, except from the deserter's report which was not 

 believed. The element of surprise was greatly lessened 

 when we began to know what to look for and to 

 recognize the sounds incident to the preparation of a gas 

 attack. 



The first attack was made with chlorine. If a gas at- 

 tack is to be made with gas clouds, the number of gases 

 available is limited. The gas must be easily compressible, 

 easily made in large quantities, and should be considerably 

 heavier than air. If to this is added the necessity of its 

 being very toxic and of low chemical reactivity, the choice 

 is practically reduced to two gases: chlorine and phos- 

 gene. Chlorine is to gas warfare what nitric acid is to 

 high explosives. Pure chlorine did not satisfy quite all 

 the requirements, as it is very active chemically and there- 



